Passive beam-deflecting apparatus

ABSTRACT

A passive beam deflector for deflecting a beam of electromagnetic energy wherein a frequency adjustable electromagnetic beam source produces a beam of energy directed at and impinging upon a Bragg scanning volume having sets of pluralities of frequency discriminating spaced parallel scattering planes oriented orthogonally to the bisector of the incident and deflected beams, whereby the beam will be deflected in a different predetermined direction depending upon the frequency of the energy.

United States Patent Robert L. Forward [72] Inventor Oxnard, Calif. [21]Appl. No. 831,533 [22] Filed June 9, 1969 [45] Patented Oct. 12, 1971[73] Assignee Hughes Aircraft Company Culver City, Calif.

[54] PASSIVE BEAM-DEFLECTING APPARATUS 14 Claims, 4 Drawing Figs. [52]U.S. 350/162 R, 350/168 [51] Int. C G021: 5/18 [50] Field of Search350/162,

[56] References Cited UNITED STATES PATENTS 3,493,288 2/1970 Kaufman eta1 350/ 163 3,485,559 12/ 1 969 Maria 350/ 162 UX ADJ I114. 8114/657/410! 704 FOREIGN PATENTS 1,088,838 10/1967 Great Britain OTHERREFERENCES Hoffman Optical information Storage in Three-DimensionalMedia Using the Lippman Technique Applied Optics Vol.7, No. 10, Oct.1968, pp. 1949-- 1954. 350 162.

Primary Examiner-John K. Corbin Attorneys-James K. Haskell and RichardJ. Rengel 1 PASSIVE BEAM-DEFLECTING APPARATUS radar or communicationsystems, and (3) in implementing au tomatic tracking of received laserbeams.

With regard to the important field of optical beam steering, prior artdevices such as modulators and deflectors were placed in theelectromagnetic energy beam generated by a laser, for example, and meanswere provided to change the effective index of refraction gradients ofthese devices using the electro-optic effect or the acousto-opticeffect. Still another technique used was the changing of the reflectancecharacteristics of the devices through modulation of surfacereflectivity or mechanical scanning. These prior art schemes all had thedisadvantage of being active devices requiring external signal and powerinputs and thus were not very efficient. Also, in many cases, the speedof operation left much to be desired and the angle over which the beamcould be scanned was undesirably limited. Further, prior devices such asfrequency scanned diffraction gratings were only capable oftwo-dimensional operation.

From the foregoing, it should be obvious that a beam scanning techniquethat overcomes the disadvantages of the prior art and provides highlyefficient and completely passive operation to speedily scan a beam overwide angles in all three dimensions would be a significant advancementin the art.

It is accordingly an object of the present invention to provide a simpleand relatively inexpensive beam-deflecting apparatus.

It is another object of the invention to provide a completely passivebeam deflection apparatus having high efficiency and fast operation.

It is still another object of the present invention to provide abeam-deflecting apparatus capable of volumetric scanning over wideangles.

The invention accomplishes the above aim by the use of a Bragg volumehaving recorded therein multiple sets of highly directional, highlywavelength dependent Bragg reflection planes to deflect the beam energygenerated by a frequency adjustable electromagnetic beam generator suchas a laser, for example. In accordance with one embodiment of theinvention, a frequency adjustable laser is utilized for generating acollimated monochromatic beam of electromagnetic energy and a Braggscanning volume is disposed in the path of the beam. The volume has setsof pluralities of scattering planes, each of which sets has a differentcharacteristic spacing between the scattering planes, and where each ofthe sets has a nonparallel relationship with others of the sets.Further, each of the sets is oriented orthogonally with respect to thebisector of the incident and the corresponding deflected beams.

The invention and specific embodiments thereof will be describedhereinafter by way of example and with reference to the accompanyingdrawings wherein like reference numerals denote like components orparts, and in which:

FIG. 1 is a schematic block diagram of apparatus for deflecting acollimated monochromatic beam of electromagnetic energy according to thepresent invention;

FIG. 2 is a schematic diagram of a frequency scanned Bragg reflectionvolume as seen in FIG. 1;

FIG. 3 is a block schematic diagram of a setup for providing a largenumber of Bragg reflection planes in a photosensitive Bragg volume usingtwo laser beams; and

FIG. 4 is a schematic block diagram of an optical radar system utilizingthe beam deflecting apparatus of FIG. 1.

With reference now to the drawings and more particularly to FIG. 1, abeam-deflecting apparatus for deflecting a collimated monochromatic beamof electromagnetic energy includes an adjustable frequency beamgenerator 11 generating a collimated monochromatic beam 13. Also, aBragg scanning volume is disposed in the beam 13 to deflect the incidentbeam energy 13 in any desired frequency-dependent direction such as theupwardly deflected beam 13a, the horizontally propagating beam 13b, orthe downwardly deflected beam 13c.

The Bragg scanning volume 15, as seen disposed in the path of the beam13, is shown in more detail in FIG. 2. Briefly stated, the volume 15includes sets of pluralities of scattering planes such as, for example,a first set of planes shown as solid lines 17 and a second set of planesshown as dashed lines 19. The Bragg volume 15 is placed in the path ofthe incident beam 13, which beam makes an angle 0, with the planes 17and 0, with the planes 19. The incident light, once striking thereflection plane, is reflected at the same angle with respect to theplane as the angle of incidence. For example, the incident beam 13 isreflected an an angle 0, by the planes 17 to produce the reflected beam130, and the energy reflected from the planes 19 is reflected at anangle (i to produce the deflected beam 13c.

The directional and frequency selectivity characteristics of the Braggvolume 15 can be derived by noting that if a wave 13 is incident at theBragg angle 0 on the reflecting planes [7, for example, having a planeseparation distance d., where the volume has a thickness T, the beam 13areflected off the m plane is mlt longer than the beam reflected oh thezeroth plane. If the incoming plane wave is not at the Bragg angle 0 butis at either of the extinction angles (0'=0+A6l2 or 0"=0A 0/2), then thebeam reflected off the m plane is (m:l))t longer than the beam reflectedoff the zeroth plane. More importantly, the beam reflected off the mplane is shifted (m/2I and the half integral wavelength shift causes them/2 beam to cancel out the zeroth beam. in addition, the m/2 +1 plane isshifted (m/2+l/m+%)A, just cancelling the beam from the first planewhich is shifted (l+l/m))t. In a similar fashion, each beam reflectedfrom a plane numbered from rn/2 to m cancels out a beam from one of theplanes numbered 0 to m/2.

The calculation of the directional selectivity A0 from the foregoingconsiderations can be expressed by A0=XIT sin 0 for A0 small but not asmall 0. Using a similar approach, the frequency selectivity may beobtained by using v the relationship Ttan6 sinQ T AMA.) Number ofSeparate Beams llmn. 3.5 570 (25 X 25) lem. 0.35 5,700 (75 x 75) 10 cm.0.035 57,000 (250 X 250) As a review of classicBragg conditions anduses, referencecan be made to any text on optics including, for example,ln-

troduction to Fourier Optics by J. W. Goodman, published byMcGraw-l-Iill, New York, i968.

From the foregoing, it can be seen that the basic concept of theinvention utilizes the Bragg reflection that occurs when light ofwavelength A is incident on a set or sets of scattering planes with aseparation d oriented at an angle 0 with respect to the beam. The angleof maximum reflection 0 is given by the relation sin MIX/2d, where n isand integer.

It has recently been found in volumetric holographic information storagesystems, many sets of Bragg reflecting planes can be set up in a volumeof photosensitive material using various combinations of object beam andreference beam orientations and colors. Since the Bragg reflectionprocess is highly wavelength and angle dependent, many images can bestored in the emulsion volume, each separated from the other on readoutthrough color and/or direction. The disclosed invention utilizes thisdemonstrated ability of photosensitive volume such as silver halideemulsions, photochromics, and photopolymers to record multiple sets ofhighly directional, highly wavelength dependent Bragg reflection planesas an electromagnetic beam deflector.

With reference to FIG. 3, the Bragg volume 15 may be fabricated fromphotosensitive materials of the type described above by setting up alarge number of Bragg reflection planes using two laser beams in a setupwhere the reflective orientation of the two beams and the photosensitivevolume are varied as indicated by the arrangement shown in the figure.Here, a single frequency laser 51 projects a laser beam 53 at a beamsplitter 55 to provide an upper beam 57 and a lower beam 59. The upperbeam 57 is first reflected by a mirror 61 to an adjustable mirror 63where it is directed at a photosensitive volume 65. At the same time,the lower beam 59 is reflected by a fixed mirror 67 toward an adjustablemirror 69 which in turn reflects the beam 59 at the volume 65. In thisarrangement, the upper beam 57 and the lower beam 59 impinge upon thevolume 65 from directly opposite directions and will produce parallelBragg scattering planes having the smallest possible spacing d, allbeing oriented in a fixed relationship in the volume 65. In order toobtain a greater spacing distance d, and thus a set of Bragg scatteringplanes responsive to a lower frequency, the volume 65 may be moved to aposition as shown by the dashed outline 65'. It will be found that thefurther the volume is moved to the right from the original position, thegreater the spacing d and the lower the responsive frequency will be.The volume 65' is also shown oriented at a different angle in order toillustrate the manner in which the angle of deflection is changed. Itshould also be noted that the volume may be rotated, prior to exposure,in an axis to provide frequency sensitive three dimensional orvolumetric deflection.

The preparation of the Bragg volume does not require the generation ofvarious frequencies since the separation between the planes d iscontrolled by the relative orientation of the two beams with respect tothe volume and the orientation of the planes in the photosensitivematerial is controlled by the orientation of the volume to the axisbetween the two laser beams. Thus, an optimum laser frequency can beused for the preparation of the Braggvolume.

In operation, the volume 15 is placed in front of the frequencyadjustable laser beam 13, and if a set of Bragg scattering planes hasbeen set up in the volume which satisfies the Bragg relation inseparation distance d and orientation for a given wavelength A, thenthere will be a strong (up to 100 percent) scattering of the incidentlaser beam 13 in the direction 20 as seen in FIG. 2. A simplifiedexample for two colors being generated by the laser 11 is shown in thisfigure. Thus, if the incident beam 13 has a long wavelength (red), thenit preferentially reflects from the widely spaced planes 17 at the angle0,, and if it has a short wavelength (blue), then it preferentiallyscatters from the closely spaced planes 19 oriented at the angle 0,.

An advantageous use of a beam-deflecting apparatus of the type describedwould be in an optical radar system as generally described in FIG. 4.Here, a laser beam 101 generated by an adjustable frequency laser 103impinges upon an optical circulator 105 from which the beam 101 ispropagated toward a Bragg volume 107 and deflected at an angle asindicated by the arrow 109, depending upon the frequency of the beam101. After being reflected from a target (not shown), target returnenergy 111 (essentially returning from the direction of the outgoingenergy) will be deflected by the Bragg volume 107 toward the opticalcirculator as indicated by arrow 1'11 and will be directed by thecirculator 105 in the form of beam 111" to a receiver 113. In thismanner, by changing the frequency of the laser beam 101 as generated bythe laser 103, a radar type energy beam may be directed in any desireddirection as provided by the orientation of the Bragg scattering planeswithin the volume 107, and the same Bragg volume 107 will direct returnenergy from that same direction toward the circulator 105 to be detectedand transformed into useful information by the receiver 1 13.

From the foregoing, it will be seen that the beam deflecting apparatusdescribed is very simple in its fabrication and utilizes known materialsand elements and therefore is economical to produce. Also, it has beenshown that the apparatus is completely passive in operation and istherefore more efiicient than the prior art active devices discussedpreviously.

In practicing this invention, any material may be employed exhibitingthe characteristics described, examples being photochromics,photopolymers, and lithium niobate, all of which have demonstratedholographic resolution capabilities.

Although several specific embodiments of the invention have been hereinillustrated, it will be appreciated that other organizations of thespecific arrangements shown may be made within the spirit and scope ofthe invention. For example, collimated monochromatic electromagneticbeam energy may be generated by devices other than lasers and thefrequency of operation of the apparatus need not be in the lightspectrum. Thus, microwave energy may be used where the Bragg volume hasBragg reflecting or scattering planes separated by the distance dresponsive to that particular microwave energy.

Accordingly, it is intended that the foregoing disclosure and drawingsshall be considered only a illustrations of the principles of thisinvention and are not to be construed in 'a limiting sense.

What is claimed is:

1. Passive beam deflecting apparatus for deflecting a collimatedmonochromatic beam of electromagnetic energy, comprising:

means for generating a collimated monochromatic beam of electromagneticenergy including means for adjusting the frequency of said energy; and

means disposed in the path of said beam responsive to said frequencyadjustment for deflecting the beam to provide beam scanning as afunction of the frequency adjustment, said latter means comprising aBragg scanning volume having sets of pluralities of scattering planes,each of said sets having a characteristic spacing between saidscattering planes, each of said sets having nonparallel relationshipswith others of said sets, and each of said sets being orientedorthogonally with respect to the bisector of the incident and thecorresponding deflected beams.

2. Beam-deflecting apparatus according to claim 1, wherein said meansfor generating a collimated monochromatic beam of electromagnetic energyis a laser including said means for adjusting the frequency of energygenerated therefrom.

3. Beam-deflecting apparatus according to claim 1, wherein said Braggscanning volume is fabricated from photosensitive materials.

4. Beam-deflecting apparatus according to claim 2, funher comprising areceiver sensitive to optical energy and optical circulator meansdisposed in the path of said beam between said laser and said Braggvolume for directing energy from said laser to said Bragg volume andenergy from said Bragg volume to said receiver.

5. A passive beam deflection system arrangement for producing a scanningbeam comprising in combinations:

means for generating a collimated monochromatic energy beam ofelectromagnetic energy having a substantial cross-sectional area toprovide a long-range collimated scanning beam of substantial energy,said means includ' ing means for varying the frequency of said energyover a predetermined frequency range;

a Bragg scanning volume disposed in the path of said beam,

said volume having a corresponding substantial cross-sectional area anda substantial thickness for passing a substantial portion of said beam,said scanning volume being responsive to said beam to provide a scanningbeam of substantial cross-sectional area and a large multiplicity ofbeams for scanning, said scanning volume comprising a large number ofsets of pluralities of scattering planes, each of said sets having acharacteristic spacing between said scattering planes.

6. The passive beam deflection system according to claim 5 in which saidenergy beam has a substantial cross sectional area which provides forretaining a collimated scanning beam for long-range scanning.

7. The passive beam deflection system according to claim 5 in which thebeam generator means and Bragg scanning volume are constructed andarranged whereby the incident beam is maintained at a constant anglecorresponding to the Bragg angle substantially throughout the operationof said system.

8. The passive beam deflection system according to claim 5 in which thethickness of said scanning volume is a thickness T of at leastapproximately 1 millimeter to provide at least approximately 570separate beams.

9. The passive beam deflection system according to claim 5 in which saidthickness T is on the order of approximately 1 millimeter to centimetersin thickness.

10. The passive beam deflection system according to claim 5 in whichsaid scanning volume comprises photochromic material.

11. The passive beam deflection system according to claim 5 in whichsaid scanning volume comprises a photopolymer material.

12. The passive beam deflection system according to claim 5 in whichsaid scanning volume comprises lithium niobate.

13. The method of passively deflecting a collimated beam ofelectromagnetic energy to produce a long-range scanning beam comprising:

generating an incident collimated beam of electromagnetic energy havinga substantial cross-sectional area to provide for maintaining collimatedbeams at said long-range;

disposing in the path of said beam, a Bragg scanning volume ofcorresponding cross-sectional area for passing at least a substantialportion of said incident beam, said scanning volume having sets ofpluralities of scattering planes, each of said sets having nonparallelrelationships with others of said sets, and each of said sets beingoriented orthogonally with respect to the bisector of the incident andthe corresponding deflected beams, so that said volume exhibits afrequency selective characteristic for different beam frequencies over arange of frequencies of said beam;

maintaining alignment of said beam and volume constant to produce ascanning beam substantially exclusively by frequency variations of saidbeam selectively deflected at different frequencies by individual onesof said sets of scattering planes;

varying the frequency of said beam over the range of frequency selectivecharacteristics of the sets of scattering planes to provide selectivereflection of said beam from different spaced sets of said scatteringplanes to produce frequency selective deflection of said beam fromindividual sets and scanning of said beam over the range of frequencyselective characteristics of said sets of pluralities of scatteringplanes.

14. The method according to claim 13 in which said incident beam anddeflected beam are of substantial cross-sectional area for producing anoutput scanning beam of substantial energy.

1. Passive beam deflecting apparatus for deflecting a coLlimatedmonochromatic beam of electromagnetic energy, comprising: means forgenerating a collimated monochromatic beam of electromagnetic energyincluding means for adjusting the frequency of said energy; and meansdisposed in the path of said beam responsive to said frequencyadjustment for deflecting the beam to provide beam scanning as afunction of the frequency adjustment, said latter means comprising aBragg scanning volume having sets of pluralities of scattering planes,each of said sets having a characteristic spacing between saidscattering planes, each of said sets having nonparallel relationshipswith others of said sets, and each of said sets being orientedorthogonally with respect to the bisector of the incident and thecorresponding deflected beams.
 2. Beam-deflecting apparatus according toclaim 1, wherein said means for generating a collimated monochromaticbeam of electromagnetic energy is a laser including said means foradjusting the frequency of energy generated therefrom. 3.Beam-deflecting apparatus according to claim 1, wherein said Braggscanning volume is fabricated from photosensitive materials. 4.Beam-deflecting apparatus according to claim 2, further comprising areceiver sensitive to optical energy and optical circulator meansdisposed in the path of said beam between said laser and said Braggvolume for directing energy from said laser to said Bragg volume andenergy from said Bragg volume to said receiver.
 5. A passive beamdeflection system arrangement for producing a scanning beam comprisingin combinations: means for generating a collimated monochromatic energybeam of electromagnetic energy having a substantial cross-sectional areato provide a long-range collimated scanning beam of substantial energy,said means including means for varying the frequency of said energy overa predetermined frequency range; a Bragg scanning volume disposed in thepath of said beam, said volume having a corresponding substantialcross-sectional area and a substantial thickness for passing asubstantial portion of said beam, said scanning volume being responsiveto said beam to provide a scanning beam of substantial cross-sectionalarea and a large multiplicity of beams for scanning, said scanningvolume comprising a large number of sets of pluralities of scatteringplanes, each of said sets having a characteristic spacing between saidscattering planes.
 6. The passive beam deflection system according toclaim 5 in which said energy beam has a substantial cross-sectional areawhich provides for retaining a collimated scanning beam for long-rangescanning.
 7. The passive beam deflection system according to claim 5 inwhich the beam generator means and Bragg scanning volume are constructedand arranged whereby the incident beam is maintained at a constant anglecorresponding to the Bragg angle substantially throughout the operationof said system.
 8. The passive beam deflection system according to claim5 in which the thickness of said scanning volume is a thickness T of atleast approximately 1 millimeter to provide at least approximately 570separate beams.
 9. The passive beam deflection system according to claim5 in which said thickness T is on the order of approximately 1millimeter to 10 centimeters in thickness.
 10. The passive beamdeflection system according to claim 5 in which said scanning volumecomprises photochromic material.
 11. The passive beam deflection systemaccording to claim 5 in which said scanning volume comprises aphotopolymer material.
 12. The passive beam deflection system accordingto claim 5 in which said scanning volume comprises lithium niobate. 13.The method of passively deflecting a collimated beam of electromagneticenergy to produce a long-range scanning beam comprising: generating anincident collimated beam of electromagnetic energy having a substantialcross-sectional area to provide for maintaining coLlimated beams at saidlong-range; disposing in the path of said beam, a Bragg scanning volumeof corresponding cross-sectional area for passing at least a substantialportion of said incident beam, said scanning volume having sets ofpluralities of scattering planes, each of said sets having nonparallelrelationships with others of said sets, and each of said sets beingoriented orthogonally with respect to the bisector of the incident andthe corresponding deflected beams, so that said volume exhibits afrequency selective characteristic for different beam frequencies over arange of frequencies of said beam; maintaining alignment of said beamand volume constant to produce a scanning beam substantially exclusivelyby frequency variations of said beam selectively deflected at differentfrequencies by individual ones of said sets of scattering planes;varying the frequency of said beam over the range of frequency selectivecharacteristics of the sets of scattering planes to provide selectivereflection of said beam from different spaced sets of said scatteringplanes to produce frequency selective deflection of said beam fromindividual sets and scanning of said beam over the range of frequencyselective characteristics of said sets of pluralities of scatteringplanes.
 14. The method according to claim 13 in which said incident beamand deflected beam are of substantial cross-sectional area for producingan output scanning beam of substantial energy.